Hydraulic interstand tension regulating and automatic gauge control system for multi-stand rolling mills

ABSTRACT

A system for controlling the delivery gauge and the interstand tensions of a tandem rolling mill by means of hydraulic cylinders located at each stand of the mill. The hydraulic cylinders, in turn, are controlled by the electrical circuitry of this invention. Gauge is controlled at the first and last stands only, while interstand tension is controlled by tensiometer feedback signals which are compared with tension reference signals to generate error signals for controlling the pressure exerted by the hydraulic cylinders and, thus, the roll gap at each stand. The speeds of all stands except the last are maintained constant. The system incorporates a deadband feature to avoid excessive operation of the hydraulic cylinders in response to noise signals.

United States Patent 1 1 Silva et al.

[ HYDRAULIC INTERSTAND TENSION REGULATING AND AUTOMATIC GAUGE CONTROLSYSTEM FOR MULTI-STAND ROLLING MILLS [111 3,744,287 1451 July 10, 19733,355,918 12/1967 Wallace 72/8 3,531,961 10/1970 Dunn 72/8 3,170,3442/1965 Marrs 72/11 3,169,421 2/ 1965 Bloodworth 72/11 [75] Inventors:Antonio V. Silva, San Paulo, Brazil; Primary Examiner-Milton Mehy GeorgeP. Gaines, Williamsville, Attorney-F. H. Henson, .1. J. Wood and R. G.N.Y. Brodahl [73] Assignee: Westinghouse Electric Corporation, ABSTRACTPittsburgh, A system for controlling the delivery gauge and the in- 22F1 d: Se L 14 1971 terstand tensions of a tandem rolling mill by meansof 1 16 p hydraulic cylinders located at each stand of the mill. PP180,374 The hydraulic cylinders, in turn, are controlled by theelectrical circuitry of this invention. Gauge is con- [52] Us CL D 72/8trolled at the first and last stands only, while interstand 51 1m. (:1B211 37/12 tehsih is hY tehsimhete feedback Signals [58] Field ofseal-11 72/s-11 which ale h1Pared with reference signals 72/16 generateerror signals for controlling the pressure exerted by the hydrauliccylinders and, thus, the rOll gap [56] References Cited at each stand.The speeds of all stands except the last are maintained constant. Thesystem incorporates a UNITED T T PATENTS deadband feature to avoidexcessive operation of the g hydraulic cylinders in response to noisesignals. 315071134 4/1970 Silva 72/8 12 Claims, 4 Drawing FiguresTERSION REFERENCE fl TO TENSION REGULATORS r TENSION STANDS TENSIONREGULATOR REGULATOR TO STAND a 2 TENSION I REGULATOR s5 2 I I0 l6 l2 01'; en T g 3224 L 531F361 wear 81%? 48 46 0| 0 DEL-IVERY GI 62 SH SPEED38 SPEED RE ufi rsiiis SPEED REGULATOR REGULATOR STA D 3&4 REGULATOR 1L1 ll '1 sPEEO 46 REFERENCE HYDRAULIC INTERSTAND TENSION REGULATING ANDAUTOMATIC GAUGE CONTROL SYSTEM FOR MULTI-STAND ROLLING MILLS BACKGROUNDOF THE INVENTION In the rolling of metal strip material in a tandemrolling mill, the various stands of the mill are set up so that eachreduces the strip by a given increment to produce the desired finalgauge at the output of the mill. For this purpose, it is desirable tomaintain the interstand tensions constant since the reduction effectedin any one stand depends not only on the roll setting of that stand, butalso on the tensions on the strip at each side of the stand.

in the past, tandem rolling mills have been provided wherein the rollgap spacing of the first stand is controlled by an automatic gaugecontrol system to compensate for changes in the thickness in the stripentering the mill. The change in the screwdown on the first mill resultsin an alteration in the reduction of the strip at that stand and, hence,a change in speed of the strip leaving the first stand and entering thesecond stand. This, in turn, alters the tension between the first andsecond stands; whereas it is desired to maintain interstand tensionconstant. Accordingly, in tandem rolling mills of this type, means areprovided for maintaining the interstand tensions constant so as toprevent variations in tension from affecting output gauge. Usually, agauge measuring device, such as an X-ray gauge, measures the thicknessof the material leaving the last stand; and if it should deviate from adesired value, the speed of the last stand is varied to compensate forany changes in gauge.

In the past, variations in roll gap spacing to vary tension and/or gaugehave been made by means of a mechanical screwdown device. Suchmechanical devices, however, have a certain inherent delay time whichmust be taken into account in any control system. It has been found thatinstead of using a mechanical screwdown to effect changes in the rollgap, better results can be obtained by using hydraulic cylinders beneaththe lower roll chocks, usually the chocks for the lower backup rolls.There are two cylinders, one on each side of the mill, which exert anupward pressure on the chocks and, hence, on the lower backup and workrolls. The hydraulic cylinders are actuated by a source of hydraulicfluid under constant pressure; and means are provided for delivering thefluid to the hydraulic cylinders whereby the rolls are urged togetherunder constant pressure. Flow control means are provided to assuresubstantially identical volumes of hydraulic fluid to each cylinder suchthat both ends of the roll will be forced upwardly or downwardly inequal amounts, thereby insuring substantially constant pressure acrossthe width of the roll gap.

SUMMARY OF THEINVENTION In accordance with the present invention, asystem is provided for regulating the tension between at least twostands in a tandem rolling mill of the type wherein a pair of hydrauliccylinders are used to exert pressure on the opposite ends of a roll ineach stand to vary the pressure exerted by the roll and the roll gapspacing of the stand. This is achieved by deriving electrical errorsignals proportional to the difference between the actual and measuredvalues of tension between each pair of stands and using these errorsignals to vary the horizontal positions of the pistons in the cylindersengaging the ends of a roll in each stand until the error signal forthat stand is reduced below a predetermined magnitude.

Corrective action is taken only after the error signal exceeds a certainmagnitude; and the error correction is terminated when the error signalfalls below a predetermined magnitude to avoid excessive operation ofthe pistons in response to noise signals. Servo systems are provided forcontrolling the positions of the pistons within the cylinders; and theseservo systems are simultaneously actuated when the aforesaid errorsignal rises above a predetermined magnitude to change the position ofboth cylinders simultaneously, thereby insuring that the forces atopposite ends of the rolls will always be the same.

Specifically, there is provided in accordance with the invention meansfor measuring the tension between at least two stands of a tandemrolling milland for producing a first electrical signal proportionalthereto, means for producing a second electrical signal proportional todesired tension between said two stands, means for comparing said firstand second electrical signals to produce an error signal, means forproducing a third electrical signal proportional to the actualhorizontal position of the piston in one of said cylinders, and meansfor producing a fourth electrical signal proportional to the actualhorizontal position of the piston in the other of the cylinders. Thethird and fourth electrical signals are stored and used in servo systemswhich control the positions of the pistons as a function of the thirdand fourth signals, respectively. When the aforesaid error signal risesabove a predetermined magnitude, the magnitude of the third and fourthelectrical signals stored in the respective servo systems are varied tothereby vary the positions of the pistons through the servo means untilthe error signal is at least partially reduced in magnitude.

In the preferred embodiment of the invention, the actual horizontalpositions of the pistons in the respective cylinders on each side of astand in a mill are measured by means of force transducers (load cells)coupled to the piston through spring means whereby the load cell outputwill represent cylinder position. The output of the load cells is thenapplied to an integrating operational amplifier which stores the valueof cylinder position and compares this with a value proportional toactual cylinder position to vary actual cylinder position if the twovalues are not the same.

When the error signal derived from a comparison of actual and desiredinterstand tension rises above a predetermined value as detected by adeadband detector, the error signal is applied to integratingoperational amplifiers in the servo systems for the respective cylindersto thereby change the stored value of cylinder position. This, whencompared with actual cylinder position, will actuate the cylinders tovary the horizontal position of the pistons therein, thereby varyingroll gap spacing to compensate for the variation in tension.

Further, in accordance with the invention, a tandem rolling mill isprovided employing hydraulic roll actuation of the type described aboveand wherein automatic gauge control of the first stand is provided aswell as the last stand. Preferably, the gauge control for the firststand is based upon a consideration of mill housing stretch; while thegauge control at the last stand is based upon a variation in deliverystrip speed. The roll gap spacings of the intermediate stands are variedto maintain tension constant as explained above.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings which form a part of this specification,and in which:

FIG. 1 is a schematic diagram of the overall control system of theinvention;

FIG. 2 is an elevational plan view of a set of rolls for one of thestands shown in FIG. 1 and illustrating the manner in which hydrauliccylinders are utilized to produce forces at the opposite ends of therolls in the stand;

FIG. 3 is a detailed schematic circuit diagram of the tension controlsystem of the invention as applied to two hydraulic cylinders for onestand of the mill; and

FIG. 4 is a plot of tension versus time showing the operation of thedeadband detector of the invention.

With reference now to the drawings, and particularly to FIG. 1, thetandem rolling mill shown includes five stands, only three of which areshown and designated as S1, S2 and S5. It will be understood, of course,that stands S3 and S4, not shown, are intermediate stands S2 and S5.Each stand, such as stand Sl, includes an outer housing which carrieswork rolls 12 and 14 backed up by backup rolls 16 and 18, respectively.The rolls 12 and 14 are carried, at their opposite ends, in suitablechocks, not shown. Similarly, the upper backup roll 16 is carried in amovable chock 20 connected to a screwdown mechanism, schematicallyillustrated at 22, the arrangement being such that the position of thebackup roll 16 can be varied upwardly or downwardly by actuation of thescrewdown mechanism 22. In most rolling mills, the screwdown mechanism22 is utilized to vary the roll gap spacing between the work rolls 12and 14; however in the mill of the present invention, it is ordinarilyutilized only to establish a nominal or preset roll gap spacing which isthen varied by hydraulic means, hereinafter described, to compensate forvariations in gauge of the strip material passing through the mill.

The ends of the lower backup roll 18 are carried in vertically movablechocks 24, only one of which is shown in FIG. 1. Each chock 24 rests, atits lower end, on a cantilever beam 26 which, in turn, engages avertically reciprocable piston 28 disposed within a cylinder 30. Betweenthe ends of the cantilever beam 26 and a shoulder at the underside ofthe housing 10 are load cells or strain gauges 32 and 34 which willproduce electrical signals proportional to the force between the ends ofthe cantilever beam 26 and the housing 10. These forces, in turn, areproportional to the displacement of the piston 28 within the cylinder30. When the piston 28 moves up, for example, the ends of the cantileverbeam 26, which acts as a spring, are compressed against the load cells32 and 34. If the cantilever beam 26 is carefully manufactured topresent a linear relationship between the force applied and theresulting compression then the following equation applies:

F=k,'d

where:

F force applied to the springs,

d distance moved by the springs, and

k, the sping constant of the springs.

This means that there will be a proportionality between the cylinderposition d and the output of the load cell. Hence, the load celloutputs, which are summed, represent cylinder position and can be usedas a feedback signal for the cylinder position regulator, hereinafterdescribed.

As shown in FIG. 2, there are two cylinders 30A and 308 on each side ofthe mill housing, as well as two pistons 28A and 288 within therespective cylinders. The pistons 28A and 288, in turn, are connectedthrough chocks 24 (FIG. 1) to the opposite ends of the lower backup roll18. Assuming that the positions of the ends of the upper backup roll 16are maintained constant by the screwdown mechanism 22, variation in thehorizontal positions of the pistons 28A and 2813 will vary the rollingforces applied at the roll gap as well as the opening of the roll gapwhich is that gap defined between the working rolls l2 and 14 andthrough which metal strip material, identified by the reference numeral36 in FIG. 1, passes. As will be seen, it is necessary that the forcesexerted by the two pistons 28A and 28B'be equal at all times.Consequently, the pressure of the hydraulic fluid introduced into thecylinders 30A and 303 must likewise be equal.

The rolls in each of the stands 81-55 are driven by drive motors DI-DScoupled to generators Gl-GS. The generators and motors, in turn, areregulated by speed regulator circuits 38-1, 33-2, 38-5, etc. Thesecircuits are controlled by a speed reference signal on lead 40 whichestablishes the speeds at each of the stands. As will be understood, thespeed of each succeeding stand must be greater than the speed of thepreceding stand since the gauge of material in each stand is less thanthat at the preceding stand.

In accordance with the present invention, all of the stands except thelast stand are driven at constant, fixed speeds. The speed of the laststand is controlled in a manner hereinafter described in order tocompensate for gauge variations at the output of the mill.

The tension between each of the stands in the mill must be maintainedconstant. At the same time, it is axiomatic that the volume of materialentering one side of the roll bite of any stand during a time intervalAt must be equal to the volume of the material leaving the roll biteduring the same time interval. This can be expressed as:

where:

G, and G are the entrance and exit gauges, respectively, and

V, and V are the speeds of the strip at the entrance and exit sides ofthe mill, respectively. Assuming that the roll gap spacing, whichdetermines G is fixed, an increase in gauge G, at the entrance side ofthe mill will result in a decrease in the entrance speed V,, since thespeed V, at the exit side of the mill is fixed by the fixed speed drivefor the mill rolls. As the speed V, at the entrance to stand S2decreases, for example, the tension in the strip 36 between stands S1and S2 will increase. This increase in tension, however, can becompensated for by increasing the roll gap spacing (i.e., G in Equation2 given above). Similarly, if the input gauge G, should decrease, thetension will decrease and the quantity V will increase. This can becompensated for by decreasing the roll gap spacing (i.e., quantity G inEquation 1). All of this can be summarized by stating that when thegauge at the input to a stand increases, the roll gap spacing must beincreased to maintain constant tension between it and the precedingstand; whereas when the gauge at the input decreases, then the roll gapspacing must likewise be decreased.

In accordance with the present invention, interstand tension between therespective stands is measured by tensiometers T2, T3, T5, etc. Thesignal from tensiometer T2 on lead 40 is compared with tension referencesignal on lead 42 by a tension regulator 44. If the two are not thesame, an error signal is generated; and if this error signal exceeds amaximum amplitude of i 3 percent, the tension regulator 44 actuates ahydraulic control circuit 46 to vary the pressure beneath the piston 28for that stand and, hence, move the piston upwardly or downwardly tovary the roll gap spacing, depending upon the polarity of the errorsignal. The roll gap spacing, however, is varied in this manner only onstands S2-S5. The first stand S1 does not incorporate a tensionregulating feature. Rather, the hydraulic control system 48 for thefirst stand is controlled by means of an entry automatic gauge controlsystem 50 connected 1 to an X-ray thickness gauge 52 at the output ofthe first stand S1 as well as to strain gauges 32 and 34. Additionally,the entry automatic gauge control system 50 is connected to a straingauge 54 between the screwdown mechanism 22 and the chock 20. Straingauge 54 produces a signal proportional to the total roll force.

While various types of automatic gauge control systems for the firststand can be used in accordance with the invention, it is preferred touse an automatic gauge control system based upon the equation:

(A m) Q/ s) where:

P total roll force as measured by load cell 54,

Q the force reading of load cells 32 and 34,

M the spring constant of the mill, and

M, the spring constant of the cantilever beam 26. The X-ray gauge 52measures the actual gauge of the strip material leaving the stand S1,and if it is not the same as the desired gauge, then an error signal isgenerated which changes the roll spacing to the hydraulic controlcircuit 48 until the error signal is zero and the gauge is constant.

As mentioned above, the speeds of all of the stands except the laststand S5 are maintained constant. At the output of stand S5, thethickness of the issuing strip material is measured by X-ray gauge 56.From gauge 56, the material passes over billy roll 58 and thence to atake-up reel 60. 1f the thickness as measured by the X-ray gauge 56 isnot equal to the desired delivery gauge, then a delivery automatic guagecontrol system 62 alters the speed of the motor D5 until the gaugeassumes the desired gauge. Remembering, again, that (7 V must equal G Va variation in G, from a desired gauge can be compensated for by acorresponding change in V Of course, at the output ,of stand S5, tensionis not a controlling factor.

With reference now to FIG. 3, the details of the electrical circuitryfor controlling the hydraulic force applied to the cylinders of a singleone of the stands are shown. The tension feedback signal on lead 40 fromtensiometer T2, for example, is applied through resistor 64 to a summingpoint 66. Also applied to the summing point 66 through resistor 68 isthe tension reference signal on lead 42. The summing point 66, in turn,is connected to the input of a proportional operational amplifier 70having a feedback path including a resistor 72. Should the tensionfeedback signal and the tension reference signal be the same, thedifference signal at point 66 will be zero. However, if the tensionfeedback and the tension reference signals are not the same, then anerror signal will appear at the summing point 66 as well as at theoutput 74 of the operational amplifier 70.

The error signal at point 74 is adapted to be applied through resistor76 and normally open contacts 78 of relay 80 to the inputs of twooperational amplifiers 82A and 823. Operational amplifies 82A, forexample, is provided with a feedback path including capacitor 84A and asecond feedback path including resistor 86A. Normally, the feedback pathwhich includes resistor 86A is connected to the input of the amplifier82A through normally closed contacts 88 of relay 80. Similarly, theoperational amplifier 82B is provided with a feedback path includingcapacitor 84B and a second feedback path including resistor 863, thissecond feedback path normally being connected to the input of theamplifier 823 through closed contacts 90 of relay 80. The outputs of thetwo load cells 32A and 34A on one side of the stand are summed at point92 and applied through resistor 94 and contacts 88 to the input ofamplifier 82A. Similarly, the outputs of the two load cells 32B and 34B.are summed at point 96 and applied through resistor 98 and contacts 90to the input of amplifier 82B. The output of amplifier 82A is appliedthrough resistor 100 to the input of proportional operational amplifier102A. Likewise, the summed strain gauge signal at point 92 is appliedthrough resistor 104A to the input of amplifier 102A. These signals areof opposite polarity such that when they are equal, the input toamplifier 102A is zero, as is its output. Amplifier 102A is providedwith a feedback path including resistor 106A.

The output of amplifier 82B is connected to a similar operationalamplifier 1028 having its input connected through resistors 100B and1048 to the output of amplifier 82B and point 96, respectively.Amplifier 1023 is provided with a feedback path including resistor 106B.

The output of amplifier 102A is adapted to be applied through normallyopen contacts 108 of relay 109 and resistor 1 10A to the input of anoperational amplifier 112A having a first feedback path including aresistor 114A. The output of operational amplifier ll2A is connected tothe coil of a flow servo valve 116A, the other end of the coil 116Abeing connected to ground through resistor 118A. The other end of coil116A is also connected through a current feedback path includingresistor 120A to the input of amplifier 112A. The flow servo controlvalve coil 116A, in turn, controls the hydraulic control circuit 46Awhich controls the position of the piston 28A in cylinder 30A. Whenpiston 28A moves upwardly or downwardly, so also will the signal atpoint 92 produced by the load cells 32A and 34A, as will be understood.

flow servo valve coil 1168 with the other end of the coil beingconnected to ground through ressitor 118B and back to the input ofamplifier 1123 to resistor 1208. The coil 1168, in turn, controls thehydraulic control circuit 468 which controls the position of piston 288in cylinder 388 on the other side of the stand.

Reverting to the operational amplifier 70, its output is also applied toa deadband detector 124 which is essentially a voltage detector. Thedeadband detector, for example, may incorporate a Zener diode which willbreak down when the error signal exceeds a certain value. Whenever thedeadband detector detects an error signal of i 3 percent, a signal onlead 126 will energize relays 80 and 109. As the error signal falls, thesignal on lead 126 will fall to zero, deenergizing relays 80 and 109whenever the error falls back to i 1. 5 percent. This isshown, forexample, in FIG. 4. When the actual tension error signal exceeds 3percent of the desired tension signal, the deadband detector 124 willenergize relays 80 and l09 .'Then, as the actual tension error signalfalls to, 1.5 percent, the relays 80 and 109 will be deenergized.

If it is assumed, for example, that the error signal at the output ofoperational amplifier 70 is :3 percent, then relays 80 and 109 areenergized: Energization of relay 80 closes contacts 78 and openscontacts 88 and 90. During this time, andsince contacts 88 and 90 areopen, the operational amplifiers 82A and 82B become integrators byvirtue of the feedback path through capacitors 84A and 84B. The errorsignal from amplifier 70, in effect, changes the value of the signalstored at the output of the operational amplifiers 82A and 828. However,as soon as the error drops below i 1.5 percent and relay 80becomes'deenergized, the tension error signal is removed from the inputsof amplifiers 82A and 828. At the same time, closing of contacts 88 and90 transforms amplifiers 82A and 823 into proportional amplifiers. Theoutput of amplifiers 82A and 828 will then remain equal to the valuethey had just before the cylinders were stopped.

When the outputs of the operational amplifiers 82A and 82B are changedin this manner, the plus and minus signals from amplifier 82A andsumming point 92, for example, will cause a proportional error outputfrom amplifier 102A which is applied through contacts 108, which are nowclosed, to the input of amplifier 112A. Amplifier 112A will now actuatecoil 116A to vary the hydraulic control circuit 46A, causing thecylinder 28 to move upwardly or downwardly. This change in pistonposition is now sensed by the load cells 32A and 34A, causing the signalat point 92 to vary until it is equal to that at the output ofoperational amplifier 82A, whereupon the corrective action ceases. Thecircuitry for cylinder 30B operates in the same manner. I

Summarizing the operation of the system, the outputs of the operationalamplifiers 82A and 828 will remain constant, just so long as the errorsignal at point 74 does not exceed 3 percent. When it does exceed 2 3percent, contacts 78 close and contacts 88 and 90 open causing acorrection to be initiated; whereupon the capacitors 84A and 84B arecharged to the correct initial value, and the pistons 28A and 288 willsmoothly start moving from their present to the new position ascommanded by amplifiers 102A and 1023. There is, of course, a similarsystem for stands S3, S4 and S5, all of which maintain the interstandtensions for the tandem mill constant.

Although the invention has been shown in connection with a certainspecific embodiment, it will be readily apparent to those skilled in theart that various changes in form and arrangement of parts may be made tosuit requirements without departing from the spirit and scope of theinvention. A

We claim as our invention:

1. in a rolling mill stand of the type wherein a pair of hydrauliccylinders having pistons therein operatively connected to roll chocksare used to exert pressure on the opposite ends of a roll to vary thepressure exerted by the roll and the roll gap spacing of the stand, the

combination of means for controlling the tension of strip materialpassing into the roll gap of the mill stand comprising:

means for measuring the tension of strip material entering said rollingmill stand and for producing a first electrical signal proportionalthereto, means for producing a second electrical signal proportional tothe desired tension of strip material entering said mill stand, meansfor comparing said first and second electrical signals to produce anerror signal,

hydraulic control means for said hydraulic cylinders,

means responsive to said error signal for actuating said hydrauliccontrol means to vary the pressure exerted by said hydraulic cylindersuntil said error signal is at least partially reduced, I

switch means for applying said error signal to said means responsive tothe error signal, said switch means being normally open, and

means for closing said normally open switch means to apply said errorsignal to the means responsive thereto only when the error signalexceeds a predetermined magnitude.

2. The combination of claim 1 including means for again opening saidswitch means when said error signal falls below a predeterminedmagnitude.

' 3. The combination of claim 2 wherein said switch means is closed whenthe error signal is equal to about i 3 percent of said first signal andsaid switch device is again opened when said error signal is less thatabout i 1.5percent of said first signal.

4. Thecombination of claim 1 wherein said means responsive to said errorsignal includes an integrating operational amplifier responsive to bothsaid error signal and a third signal porportional to the positions ofsaid pistons within the hydraulic cylinders, and proportionaloperational amplifier means connected to the output of said integratingamplifier means and connecting said integrating amplifier means to saidhydraulic control means.

5. The combination of claim 4 including spring means connected to saidpistons within the hydraulic cylinders, and load cell means interposedbetween said spring means and a stationary point, said load cell meansproducing said third signal proportional to the horizontal position ofthe pistons within said cylinders.

6. The combination of claim wherein there are load cell means for bothhydraulic cylinders on opposite sides of the mill, and includingseparate operational amplifier means and hydraulic control means foropposite sides of the mill, said error signal being simultaneouslyapplied to both of said operational amplifier means.

7. In a gauge control system for a tandem rolling mill of the typewherein a pair of hydraulic cylinders having pistons therein are used toexert pressure on the opposite ends of a roll in each stand of the millto vary the pressure exerted by the roll and the roll gap spacing of thestand, the combination of means for controlling the interstand tensionof strip material passing between at least two stands of the millcomprising:

means for measuring the tension between said two stands of the mill andfor producing a first electrical signal proportional thereto,

means for producing a second electrical signal proportional to desiredtension between said two stands,

means for comparing said first and second electrical signals to producean error signal,

means for producing a third electrical signal proportional to theposition of the piston in one of said cylinders,

means for producing a fourth electrical signal porportional to theposition of the piston in the other of said cylinders,

means for storing said third and fourth electrical signals,

servo means for controlling the position of said pistons as a functionof said third and fourth signals respectively, and

means operable when said error signal rises above a predeterminedmagnitude for simultaneously varying the magnitudes of said third andfourth signals to thereby vary the positions of said pistons throughsaid servo means until said error signal is at least partially reducedin magnitude.

8. The combination of claim 7 wherein said' means operable when saiderror signal rises above a predetermined magnitude comprises a deadbanddetector, and means operatively connected to said deadbeand detector forapplying said error signal to said servo means.

9. The combination of claim 7 including means for measuring the gauge ofstrip material at the output of the first stand in said tandemgmill andfor actuating the hydraulic cylinders of said firststand to change itsroll gap spacing when the measured gauge departs from a desired value.

10. The combination of claim 9 including means for measuring the gaugeof strip material issuing from the last stand of said tandem mill, andmeans coupled to said last-mentioned measuring means for varying thespeed of said last stand when the measured gauge of strip materialissuing from the last stand departs from a desired value. I

11. The combination of claim 10 including means for maintaining thespeed at all stands in the mill constant with the exception of said laststand.

12. In a tandem rolling mill for strip material of the type wherein gageis controlled by manipulating at least the first or last stand in themill while tension is maintained essentially constant at the input andoutput sides of at least one intermediate stand of the mill and whereina pair of hydraulic cylinders having pistons. therein operativelyconnected to roll chocks are used to exert pressure on the opposite endsof a roll in said intermedaite stand to thereby vary the pressureexerted by the roll and the roll gap spacing of the stand, thecombination of means for controlling the tension of strip materialpassing into the roll gap of said intermediate stand comprising:

means for measuring the tension of strip material entering saidintermediate stand and for producing a first electrical signalproportional thereto,

means for producing a second electrical signal proportional to thedesired tension of strip material entering said intermediate stand,

means for comparing said first and second electrical signals to producean error signal,

hydraulic control means for said hydraulic cylinders,

and

means responsive to said error signal for actuating said hydrauliccontrol means to vary the pressure exerted by said hydraulic cylindersuntil said error signal is at least partially reduced.

1. In a rolling mill stand of the type wherein a pair of hydrauliccylinders having pistons therein operatively connected to roll chocksare used to exert pressure on the opposite ends of a roll to vary thepressure exerted by the roll and the roll gap spacing of the stand, thecombination of means for controlling the tension of strip materialpassing into the roll gap of the mill stand comprising: means formeasuring the tension of strip material entering said rolling mill standand for producing a first electrical signal proportional thereto, meansfor producing a second electrical signal proportional to the desiredtension of strip material entering said mill stand, means for comparingsaid first and second electrical signals to produce an error signal,hydraulic control means for said hydraulic cylinders, means responsiveto said error signal for actuating said hydraulic control means to varythe pressure exerted by said hydraulic cylinders until said error signalis at least partially reduced, switch means for applying said errorsignal to said means responsive to the error signal, said switch meansbeing normally open, and means for closing said normally open switchmeans to apply said error signal to the means responsive thereto onlywhen the error signal exceeds a predetermined magnitude.
 2. Thecombination of claim 1 including means for again opening said switchmeans when said error signal falls below a predetermined magnitude. 3.The combination of claim 2 Wherein said switch means is closed when theerror signal is equal to about + or - 3 percent of said first signal andsaid switch device is again opened when said error signal is less thatabout + or - 1.5 percent of said first signal.
 4. The combination ofclaim 1 wherein said means responsive to said error signal includes anintegrating operational amplifier responsive to both said error signaland a third signal proportional to the positions of said pistons withinthe hydraulic cylinders, and proportional operational amplifier meansconnected to the output of said integrating amplifier means andconnecting said integrating amplifier means to said hydraulic controlmeans.
 5. The combination of claim 4 including spring means connected tosaid pistons within the hydraulic cylinders, and load cell meansinterposed between said spring means and a stationary point, said loadcell means producing said third signal proportional to the horizontalposition of the pistons within said cylinders.
 6. The combination ofclaim 5 wherein there are load cell means for both hydraulic cylinderson opposite sides of the mill, and including separate operationalamplifier means and hydraulic control means for opposite sides of themill, said error signal being simultaneously applied to both of saidoperational amplifier means.
 7. In a gauge control system for a tandemrolling mill of the type wherein a pair of hydraulic cylinders havingpistons therein are used to exert pressure on the opposite ends of aroll in each stand of the mill to vary the pressure exerted by the rolland the roll gap spacing of the stand, the combination of means forcontrolling the interstand tension of strip material passing between atleast two stands of the mill comprising: means for measuring the tensionbetween said two stands of the mill and for producing a first electricalsignal proportional thereto, means for producing a second electricalsignal proportional to desired tension between said two stands, meansfor comparing said first and second electrical signals to produce anerror signal, means for producing a third electrical signal proportionalto the position of the piston in one of said cylinders, means forproducing a fourth electrical signal proportional to the position of thepiston in the other of said cylinders, means for storing said third andfourth electrical signals, servo means for controlling the position ofsaid pistons as a function of said third and fourth signalsrespectively, and means operable when said error signal rises above apredetermined magnitude for simultaneously varying the magnitudes ofsaid third and fourth signals to thereby vary the positions of saidpistons through said servo means until said error signal is at leastpartially reduced in magnitude.
 8. The combination of claim 7 whereinsaid means operable when said error signal rises above a predeterminedmagnitude comprises a deadband detector, and means operatively connectedto said deadbeand detector for applying said error signal to said servomeans.
 9. The combination of claim 7 including means for measuring thegauge of strip material at the output of the first stand in said tandemmill and for actuating the hydraulic cylinders of said first stand tochange its roll gap spacing when the measured gauge departs from adesired value.
 10. The combination of claim 9 including means formeasuring the gauge of strip material issuing from the last stand ofsaid tandem mill, and means coupled to said last-mentioned measuringmeans for varying the speed of said last stand when the measured gaugeof strip material issuing from the last stand departs from a desiredvalue.
 11. The combination of claim 10 including means for maintainingthe speed at all stands in the mill constant with the exception of saidlast stand.
 12. In a tandem rolling mill for strip material of the typewherein gage is controlled by manipulating at least the first or laststand in the miLl while tension is maintained essentially constant atthe input and output sides of at least one intermediate stand of themill and wherein a pair of hydraulic cylinders having pistons thereinoperatively connected to roll chocks are used to exert pressure on theopposite ends of a roll in said intermediate stand to thereby vary thepressure exerted by the roll and the roll gap spacing of the stand, thecombination of means for controlling the tension of strip materialpassing into the roll gap of said intermediate stand comprising: meansfor measuring the tension of strip material entering said intermediatestand and for producing a first electrical signal proportional thereto,means for producing a second electrical signal proportional to thedesired tension of strip material entering said intermediate stand,means for comparing said first and second electrical signals to producean error signal, hydraulic control means for said hydraulic cylinders,and means responsive to said error signal for actuating said hydrauliccontrol means to vary the pressure exerted by said hydraulic cylindersuntil said error signal is at least partially reduced.